This invention generally relates to the area of signalling protocol systems. In particular, this invention relates to the area of a signalling protocol used with a paging system having a transmitter and a plurality of portable selective call receivers such as paging receivers.
A signalling protocol developed by Great Britain's Post Office Code Advisory Group (POCSAG) is widely used in paging systems. Also, the Golay Sequential Code (GSC) has enjoyed a number of years of satisfactory use in paging systems the world over. However, the expanding market for paging receivers now requires a signalling system which communicates message information with less overhead than prior art systems.
The POCSAG protocol has overhead which diminishes its ability to communicate information. POCSAG reserves one information bit in every one of the 32,21 BCH message information words to establish the type of information included therein. The bit being in a first logic state indicates address information, and the second state indicates message information is contained within the word. Thus, of 21 information bits in the 32,21 word, one bit, or about 5% of the information, is used simply to indicate the type of information contained within the remaining 20 bits. Being as this bit does not contribute to the actual message information, it is overhead. A second type of overhead is a synchronization word inserted between every sixteen message information words. The synchronization word was used in order to maintain synchronization during the transmission of the protocol. However, advances in the state of the art in synchronization of paging receivers has eliminated the requirement for the sync code. Thus, the sync code occurring as one of every seventeen words, about 6% of the words, is no longer necessary and is also considered overhead. A third type of overhead occurs with the transmission of data messages. Of the data messages, telephone numbers are most often communicated. A formatted ten-digit telephone number uses one address word, three data words and an idle codeword. The idle codeword communicates no message information but acts as a separation signal between data messages. Thus, of the five words, one address, three data and one idle, the idle codeword provides no message information and is thus considered overhead. Thus, the overhead of the idle codeword can typically be 20%. Therefore much efficiency can be gained with an improved signalling protocol.
The POCSAG protocol additionally suffers from the lack of ability to indicate in advance the occurrence of addresses. Since an address can occur almost anywhere within a transmission, paging receivers are required to search for address information when message information is transmitted. Furthermore, the POCSAG protocol suffers from a finite probability, that bit errors occurring in the message may cause the message information to be falsely interpreted. This is commonly considered a "false". It is desirable to provide for the reduction of falsely interpreting message information.
The GSC protocol has similar overhead signals and suffers from similar protocol problems.